An optical resonant frequency converter is used to produce in a particularly efficient manner from a laser beam involving a fundamental wavelength, referred to hereinafter as the fundamental wave, by non-linear conversion in a suitable non-linear crystal, a laser beam involving a higher and in particular doubled frequency, referred to hereinafter as the converted beam. The technology of non-linear conversion is used whenever a suitable active laser material is not available for the direct production of the desired wavelength. Because of the long service life and high level of efficiency semiconductor lasers and diode-pumped solid state lasers (DPSS lasers) are nowadays increasingly used to produce continuous laser light in the red and infrared spectral range. Shorter wavelengths can then usually be produced by non-linear conversion. Conversion can take place in a plurality of steps. In the case of DPSS lasers the first conversion step is effected to produce visible laser radiation frequently in the laser resonator itself (‘intracavity doubling’). The further conversion to still shorter wavelengths is preferably implemented outside the laser resonator. Particularly in the case of non-linear production of continuous UV laser light, resonant frequency doubling in an external resonator plays a significant part as the crystal materials available for that wavelength range have only low non-linear coefficients and therefore non-resonant conversion is too inefficient for practical use. The combination of an intracavity-frequency-doubled DPSS laser or a semiconductor laser with a resonant frequency converter affords a laser source for continuous UV laser light which has many different applications in the semiconductor, consumer electronics and telecommunications industry.
The principle of resonant frequency doubling has long been known (see for example Ashkin et al ‘Resonant Optical Second Harmonic Generation and Mixing’, Journal of Quantum Electronics, QE-2, 1966, page 109; or M Brieger et al ‘Enhancement of Single Frequency SHG in a Passive Ring Resonator’, Optics Communications 38, 1981, page 423). In that case the fundamental wave is coupled into an optical resonator comprising mirrors and which is resonantly tuned to the frequency of the fundamental wave. For that purpose the optical length of the resonator is so set by means of a suitable device that it is an integral multiple of the fundamental wavelength. If the losses in the resonator are low and the coupling-in mirror is of a partially transparent nature with a suitably selected degree of reflection, then an enhancement in resonance takes place, that is to say the power of the light wave circulating in the resonator is greater than the power of the fundamental wave which was radiated in from the exterior. The degree of reflection R of the coupling-in mirror is at an optimum when the following applies:R=1−Vwherein V denotes the relative losses of the circulating light wave in a revolution in the resonator, hereinafter referred to as resonator losses. Under that condition referred to as ‘impedance matching’ the enhancement factor is:A=1/V,that is to say the light wave circulating in the resonator has A-times the power of the light wave which is radiated in. In practice enhancement factors of between 100 and 200 are achieved.
Disposed in the resonator is a non-linear crystal through which the circulating fundamental wave is radiated and which, by non-linear conversion, produces a second light wave at double the frequency, which is coupled out of the resonator by a resonator mirror which is transparent at that double frequency.
So that production of the converted beam takes place with a usable level of efficiency, phase matching must occur in the non-linear crystal, that is to say the refractive index of the crystal at the fundamental wavelength must be of the same magnitude as its refractive index at the converted wavelength. Phase matching can be effected by angle tuning (critical phase matching) or by temperature matching (non-critical phase matching). In the case of non-critical phase matching the efficiency of frequency conversion is generally higher and the beam profile of the converted beam is of higher quality, that is to say closer to the desired Gaussian beam shape. The crystal materials available at the present time however permit the use of non-critical phase matching only for a few, narrow wavelength ranges. In particular at the present time no crystal material exists, with which laser light can be produced in the low UV range with non-critical phase matching.
As the power of the converted beam is proportional to the square of the power density of the fundamental wave, the level of efficiency of non-linear conversion is increased when the fundamental wave is focused in the non-linear crystal. Therefore the resonator mirror is generally provided with spherically curved surfaces so that a beam waisting effect is formed in the middle of the crystal. The power density in the crystal can be increased by reducing the beam waisting. The divergence of the beam in the crystal, which increases at the same time, reduces however the level of conversion efficiency in crystals which are substantially longer than the waist region (Raleigh length) of the beam. There is therefore an optimum size of the beam waist which can be adjusted by suitable selection of the spacings and the radii of curvature of the resonator mirrors.
Due to the resonant enhancement of the fundamental wave power in the resonator the level of conversion efficiency is increased in comparison with a non-resonant arrangement, by some orders of magnitude. Thus for example the levels of conversion efficiency which can be achieved with the present-day state of the art, for producing UV laser radiation at 266 nm, are between 20% and 40% when a fundamental wave power of between 1W and 5W is available. In that power range saturation of efficiency already occurs so that development endeavours are unnecessary at least in regard to conversion efficiency. When using fundamental wave lasers in the power range of between 10 mW and 100 mW which are suitable for building particularly compact UV lasers however the conversion efficiency of a frequency converter in accordance with the state of the art is unsatisfactorily low as that power range still involves a quadratic dependency on the fundamental wave power.
In general terms adverse effects in terms of the output power of a laser with frequency converter occur by virtue of various phenomena discussed hereinafter.
The object of the invention is to provide a frequency converter which substantially avoids power impairments.